The present invention relates to a grinding machine, a grinding method, a method of fabricating semiconductor devices, and a method of fabricating semiconductor thin-film substrates. For example, the invention relates to a grinding machine for grinding surfaces of semiconductor wafers, to a method of grinding surfaces of semiconductor wafers, to a method of fabricating semiconductor devices by making use of steps for planarizing a substrate surface and a thin film formed on the planarized substrate surface, and to a method of fabricating semiconductor thin-film substrates having a quite thin semiconductor single-crystal film for use in semiconductor integrated circuits, semiconductor devices, or display devices.
Where the surface of a desired workpiece is machined, various machines and various methods can be used, depending on the material of the workpiece, on the shape, and on the surface state to be obtained after the machining process. Where the surface of a semiconductor wafer is machined, it is common practice to employ lapping. In a lapping operation, free abrasive grains are inserted between a platen and a semiconductor wafer that is a workpiece. The platen is made to make a planetary motion relative to the workpiece while maintaining the platen in contact with the workpiece. In this way, the semiconductor wafer surface is machined.
Another machining method is grinding using a cup-shaped grinding wheel as shown in FIG. 6. When a grinding operation is performed with the cup-shaped grinding wheel, a semiconductor wafer 12 is placed opposite to the outer surface of the grinding wheel 11 in such a way that the center of rotation of the wafer 12 substantially agrees with the outer surface of the grinding wheel 11. The grinding wheel 11 and the wafer 12 are rotated in opposite directions as indicated by the arrows D and E, respectively. Under this condition, the cup-shaped grinding wheel 11 is fed into the wafer in the direction indicated by the arrow F. In this manner, the surface of the semiconductor wafer 12 is machined.
Where a curved surface such as of a lens or reflecting mirror is machined, a grinding operation is carried out, using an extremely thin grinding wheel 13 as shown in FIGS. 7A and 7B. As shown in FIG. 7B, the extremely thin grinding wheel 13 comprises an extremely thin (2 to 3 mm) disklike grinding wheel whose outer surface has been shaped like the letter "R". This grinding wheel 13 and a workpiece 16 are rotated. The grinding wheel 13 is fed into the workpiece in the direction indicated by the arrow H while reciprocating a grinding wheel shaft 14 in the direction indicated by the arrow G of FIG. 7A. In this way, the surface of the workpiece 12 is machined. In this machining process, the extremely thin grinding wheel 13 makes contact with the workpiece 12 at a point. Therefore, a desired curved surface can be obtained.
On the other hand, chemical-mechanical polishing (CMP) has attracted attention as a new planarizing technique for semiconductor fabrication processes. FIG. 9 is a cross section of a semiconductor wafer being polished, schematically illustrating the CMP technique. The silicon wafer, indicated by 36, is held to a suction platen 35 and rotated as indicated by the arrow. The wafer is kept in contact with polishing cloth 37 on the surface of a polishing machine. That is, CMP is a process making use of both mechanical processing and chemical etching to planarize the surface of the silicon wafer without damaging it. The mechanical processing utilizes both the polishing cloth 37 and a compound in the polishing fluid. The chemical etching makes use of a chemical solvent contained in the polishing fluid. Accordingly, it is necessary to select the polishing fluid according to the object to be polished. In the CMP, reaction products formed on the wafer surface are removed generally by mechanical polishing between colloidal silica and the polishing cloth and thus chemical reactions are promoted. That is, CMP processes the desired workpiece by mechanochemical reactions. For example, where the polished workpiece is silicon, NaOH which is the main constituent of the abrasive cooperates with the silicon surface layer to form a layer of the reaction product Na.sub.2 SiO.sub.3, as given by the following chemical reaction: EQU Si+2NaOH+H.sub.2 O.fwdarw.Na.sub.2 SiO.sub.3 +2H.sub.2
This reaction product 38 is removed by mechanical polishing of colloidal silica (compound) and polishing cloth to expose a new silicon surface. As a result, the chemical reaction on the surface is continued. Thus, the polishing process is made to progress. The temperature at the surface of the wafer is elevated by frictional heat and so the chemical reaction is accelerated.
Some methods are known to form a single-crystal thin film in a semiconductor thin-film substrate. In one method, gas is supplied onto a crystal substrate and a single-crystal film is epitaxially grown. In another method, a thick semiconductor single-crystal substrate is bonded to a second substrate. Then, the first-mentioned semiconductor single-crystal substrate is polished to form a thin film. Also, the aforementioned chemical-mechanical polishing (CMP) has attracted attention as a new planarizing technique for semiconductor manufacturing processes.
The conventional grinding machines and methods using these conventional machines use the several methods described above. However, these have the following drawbacks.
In lapping, if the workpiece is a brittle material such as a semiconductor wafer, innumerable cracks are produced on the surface of the workpiece because of brittle fracture, leaving a deep layer whose property has been modified by the processing. Therefore, it has been heretofore necessary to etch away the modified layer with an acid or the like after the lapping.
Furthermore, the cracks produced by the lapping have nonuniform depths. This increases the amount of the modified layer to be etched away. This in turn greatly deteriorates the flatness. Accordingly, after the etching step, polishing is necessary to modify the shape. In this way, the efficiency of the machining process is quite low. Moreover, the planetary motion makes the machine bulky.
In addition, in the case of grinding using the cup-shaped grinding wheel, the cup-shaped grinding wheel 11 provides a large area in contact with the workpiece 12 such as a semiconductor wafer, as shown in FIG. 6. Hence, the machining load is large. Therefore, in order to improve the accuracy of the shape, it is necessary to enhance the rigidity. This also increases the size of the machine. Further, if the contact area between the cup-shaped grinding wheel 11 and the workpiece 12 is large, then each individual abrasive grain makes contact with the workpiece for a long time. In consequence, the load imposed on the abrasive grain per rotation of the grinding wheel is large. As a result, abrasive grains tend to come off and to cause rapid wear. When abrasive grains come off, they are dragged on the surface of the workpiece, resulting in cracks. Furthermore, abrasive dust is not efficiently removed, so that the dust is dragged. This also leads to cracks.
In machining processes using the extremely thin grinding wheel 13 shown in FIG. 7, the grinding wheel 13 provides a small area in contact with the workpiece 12 such as a semiconductor wafer. Hence, the machining efficiency is very low. Furthermore, the grinding wheel makes contact with the workpiece at a point and so it is necessary to very accurately align the center position (machined point) of the grinding wheel 13 indicated by the dot-and-dash line P in FIG. 7B with the position of the center of rotation of the workpiece indicated by the phantom line Q. However, if the center position of the grinding wheel 13 slightly deviates from the center of rotation of the workpiece, then some part of the central portion of the workpiece will be left unground.
Where CMP is applied to semiconductor device fabrication processes, the following problems occur.
(1) Where a large area is machined by CMP, the central portion is dished out. Therefore, the surface of the semiconductor thin-film substrate has unevenness on the order of 0.5 .mu.m. PA1 (2) The abrasive used in CMP machining is an alkali solution such as NaOH or KOH containing colloidal silica or the like. Therefore, the abrasive or its reaction products are left on the wafer surface. The remaining abrasive or reaction products deteriorate the device characteristics or production yield. PA1 (3) In the case of CMP machining, the whole wafer surface is machined. Therefore, it is difficult to detect the endpoint of the machining process. This makes it impossible to accurately control the film thickness. PA1 (4) Since a chemical etching is conducted, it is necessary to select the abrasive according to the object to be polished. PA1 (1) The underlying substrate must be a crystal. Furthermore, this is limited to substrates having lattice constants substantially equal to that of the epitaxially grown thin film. PA1 (2) A high temperature exceeding about 1000.degree. C. is necessary for the epitaxial growth. Lowering the temperature deteriorates the crystallinity. PA1 (3) When the film is epitaxially grown, anomalous growth is observed in some portions. This makes it impossible to achieve a uniform film over the whole substrate.
When a single-crystal film is epitaxially grown for fabrication of a semiconductor thin-film substrate, the following problems occur:
Further, where a semiconductor thin-film substrate is formed by CMP machining, the same drawbacks are produced as occurring when CMP machining is applied to the aforementioned semiconductor device fabrication method.